My opponent had a Gloomlake Virge and a Reflecting Pool as their only mana sources and used them to cast The Terminus of Return. Gloomlake virge could only produce blue at the time since they did not control an island or swamp, so their Reflecting Pool should have only been able to produce blue mana at that time, but it apparently added the black needed to cast the spell.

According to CR 106.7:
Some abilities produce mana based on the type of mana another permanent or permanents “could
produce.” The type of mana a permanent could produce at any time includes any type of mana that
an ability of that permanent would produce if the ability were to resolve at that time, taking into
account any applicable replacement effects in any possible order. Ignore whether any costs of the
ability could or could not be paid.